| So the superconductivity the magnets are guaranteed, they have to be cooled down to 1.9 K (-271.3 Â ° C). The world's largest cooling system was designed to make this possible. It takes 1 month to warm up or cool down a sector of the LHC ring. The magnets of the LHC storage ring and the large detectors are first pre-cooled with 6000 tons of liquid nitrogen to 80 Kelvin (-193Â ° C) and then with a total of 140 tons of liquid helium, close to absolute zero, to 1.9 Kelvin (-271.3Â ° C) cooled. The dipole magnets, which are up to 15 meters long, shrink by several centimeters. Special connection buffers, valves and approx. 40,000 seals are necessary so that the entire system remains tight. Helium is the only substance suitable to meet the high requirements of cooling. Below 2.2 Kelvin, helium becomes helium II. This state has a thermal conductivity that is around a million times higher than that of normal helium. In addition, Helium II is superfluid, i.e. it flows without any friction and passes through the smallest of openings. Therefore, the handling of Helium II requires special materials that can meet these requirements.|
|Current average temperature of all sectors||Source: CERN|
A sophisticated distribution network was installed so that all superconducting magnets are constantly supplied with helium. On the earth's surface, special cooling compressors, each with an output of 18,000 watts, are installed in 8 places above the ring. Each delivers two streams of helium; a gaseous stream with a temperature of 50 Kelvin and a liquid one with 4.5 Kelvin. The helium streams are then sent to a depth of about a hundred meters. Underground, the cryogenic helium is fed to the ring elements via a complex distribution system and flows through magnet coils and cooling jackets.
|18 KW compressor system||Source: CERN|
|Locations of the 8 main compressors||Source: CERN|
|Helium tanks||Source: CERN|
|Installation of the helium distribution pipes in the LHC tunnel||Source: CERN|
Cold Compression System (CCS)
The Cold Compression System (CCS) ensures even lower temperatures. In order to reach a temperature of 1.8 Kelvin (-271.4 Â ° C), the helium pressure is reduced to an absolute value of 16 millibars by means of multi-stage turbo compressors. In addition, the 1.4% incline of the LHC ring had to be taken into account. Through them, the coolant has to be pumped partly with and partly against gravity. This is problematic because it results in pressure fluctuations, which in turn influence the temperature of the helium. In addition, the temperature within one of the 15-meter-long magnets must not vary by more than 75 K while it is warming up or cooling down. The distribution of the helium within the magnet modules is regulated by special valves for extremely low temperatures. The valves are moved by actuators operated by compressed air, as the radiation exposure in the magnets could affect or even destroy electrically operated valves. The superconducting magnets themselves are surrounded by stationary helium II, which absorbs the heat and dissipates it via a line through which He II flows.
|Cold Compression System||Source: CERN|
|Helium distribution within a magnetic module||Source: CERN|